1. Field
The present invention relates generally to communications, and more specifically to a novel and improved method and apparatus for power control of serving and non-serving base stations.
2. Background
Wireless communication systems are widely deployed to provide various types of communication such as voice and data. These systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), or some other multiple access techniques. A CDMA system provides certain advantages over other types of systems, including increased system capacity.
A CDMA system may be designed to support one or more CDMA standards such as (1) the “TIA/EIA-95-B Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System” (the IS-95 standard), (2) the standard offered by a consortium named “3rd Generation Partnership Project” (3GPP) and embodied in a set of documents including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214 (the W-CDMA standard), (3) the standard offered by a consortium named “3rd Generation Partnership Project -2” (3GPP2) and embodied in “TR-45.5 Physical Layer Standard for cdma2000 Spread Spectrum Systems” (the IS-2000 standard), and (4) some other standards.
In the above named standards, the available spectrum is shared simultaneously among a number of users, and techniques such as power control and soft handoff are employed to maintain sufficient quality to support delay-sensitive services, such as voice. Data services are also available. More recently, systems have been proposed that enhance the capacity for data services by using higher order modulation, very fast feedback of Carrier-to-Interference ratio (C/I) from the mobile station, very fast scheduling, and scheduling for services that have more relaxed delay requirements. An example of such a data-only communication system using these techniques is the High Data Rate (HDR) system that conforms to the TIA/EIA/IS-856 standard (the IS-856 standard).
In contrast to the other above named standards, an IS-856 system uses the entire spectrum available in each cell to transmit data to a single user at one time, wherein the user is selected based on link quality and other considerations, such as data pending, etc. In so doing, the system spends a greater percentage of time sending data at higher rates when the channel is good, and thereby avoids committing resources to support transmission at inefficient rates. The net effect is higher data capacity, higher peak data rates, and higher average throughput.
Systems may may incorporate support for delay-sensitive data, such as voice channels or data channels supported in the IS-2000 standard, along with support for packet data services such as those described in the IS-856 standard. One such system is described in a proposal submitted by LG Electronics, LSI Logic, Lucent Technologies, Nortel Networks, QUALCOMM Incorporated, and Samsung to the 3rd Generation Partnership Project 2 (3GPP2). The proposal is detailed in documents entitled “Updated Joint Physical Layer Proposal for 1xEV-DV”, submitted to 3GPP2 as document number C50-20010611-009, Jun. 11, 2001; “Results of L3NQS Simulation Study”, submitted to 3GPP2 as document number C50-20010820-011, Aug. 20, 2001; and “System Simulation Results for the L3NQS Framework Proposal for cdma2000 1xEV-DV”, submitted to 3GPP2 as document number C50-20010820-012, Aug. 20, 2001. These, and related documents generated subsequently, such as Revision C of the IS-2000 standard, including C.S0001.C through C.S0006.C, are hereinafter referred to as the 1xEV-DV proposal.
A system such as the one described in the 1xEV-DV proposal generally comprises channels of four classes: overhead channels, dynamically varying IS-95 and IS-2000 channels, a Forward Packet Data Channel (F-PDCH), and some spare channels. The overhead channel assignments vary slowly, they may not change for months. They are typically changed when there are major network configuration changes. The dynamically varying IS-95 and IS-2000 channels are allocated on a per call basis or are used for IS-95, or IS-2000 Release 0 through B packet services. Typically, the available base station power remaining after the overhead channels and dynamically varying channels have been assigned is allocated to the F-PDCH for remaining data services. The F-PDCH may be used for data services that are less sensitive to delay while the IS-2000 channels are used for more delay-sensitive services.
The F-PDCH, similar to the traffic channel in the IS-856 standard, is used to send data at the highest supportable data rate to one user in each cell at a time. In IS-856, the entire power of the base station and the entire space of Walsh functions are available when transmitting data to a mobile station. However, in the proposed 1xEV-DV system, some base station power and some of the Walsh functions are allocated to overhead channels and existing IS-95 and cdma2000 services. The data rate that is supportable depends primarily upon the available power and Walsh codes after the power and Walsh codes for the overhead, IS-95, and IS-2000 channels have been assigned. The data transmitted on the F-PDCH is spread using one or more Walsh codes.
In the 1xEV-DV proposal, the base station generally transmits to one mobile station on the F-PDCH at a time, although many users may be using packet services in a cell. (It is also possible to transmit to two or more users, by scheduling transmissions for the two or more users and allocating power and/or Walsh channels to each user appropriately.) Mobile stations are selected for forward link transmission based upon some scheduling algorithm.
In a system similar to IS-856 or 1xEV-DV, scheduling is based in part on channel quality feedback from the mobile stations being serviced. For example, in IS-856, mobile stations estimate the quality of the forward link and compute a transmission rate expected to be sustainable for the current conditions. The desired rate from each mobile station is transmitted to the base station. The scheduling algorithm may, for example, select a mobile station for transmission that supports a relatively higher transmission rate in order to make more efficient use of the shared communication channel. As another example, in a 1xEV-DV system, each mobile station transmits a Carrier-to-Interference (C/I) estimate as the channel quality estimate on the Reverse Channel Quality Indicator Channel or R-CQICH. The scheduling algorithm is used to determine the mobile station selected for transmission, as well as the appropriate rate and transmission format in accordance with the channel quality. A variety of scheduling algorithms may be implemented, such as the Proportional-Fair algorithm detailed in U.S. Pat. No. 6,229,795.
In such a system, a mobile station receives forward link data from a serving base station. As described, reverse link feedback from a mobile station to the serving station may be used for forward link scheduling and transmission, and may also be used for serving base station power control. Soft handoff in the above listed systems is not used for such forward link data transmission. That is, a mobile station will not receive the Forward Packet Data Channel, or F-PDCH, from more than one base station. However, a mobile station may be in soft handoff on the reverse link with one or more non-serving base stations and/or sectors to provide reverse link switching diversity. Since the path loss characteristics of each path between a mobile station and multiple base stations will, in general, be different, the serving base station power control mechanism for its transmission to the mobile station may not be suitable for non-serving base stations for the same mobile station. To optimize system capacity, it is desirable for the reverse link as well as any forward link signaling between a mobile station and non-serving base stations to be power controlled. However, maintaining a power control loop for each non-serving base station may use excessive resources on the reverse link. There is therefore a need in the art for power control of serving and non-serving base stations.